Currently, a common feature of optical access technologies is that optical signals between an optical line terminal (OLT) and an optical network unit (ONU) are distributed through an optical splitter, and upstream and downstream carriers separately adopting different wavelengths are transmitted for data transmission.
An optical network system in the prior art has large optical attenuation, and a conventional passive optical network (PON) technology is limited on aspects such as the transmission distance and the split ratio. Each ONU obtain a limited upstream bandwidth, so that the demand for high-bandwidth services such as high resolution digital service is hard to be satisfied.
For the situation of the optical network system in the prior art, problems of the PON technology can be solved by introducing a wavelength division multiplex (WDM) technology in the access network. As shown in FIG. 1, each ONU is assigned with two different wavelengths for transmitting upstream and downstream carriers each. An OLT is placed in a central office (CO) as shown in FIG. 1. The OLT employs a multi-wavelength light source. The data transmitted to an ONU1, an ONU2, . . . , and an ONUn in a downstream direction is sent after being modulated on optical carriers having wavelengths λ1, λ2, . . . , λn respectively. Signals are distributed through a passive remote node (RN) to destination ONUs so as to implement the downstream transmission. During the upstream transmission, a channel competition problem among the ONUs is solved by means of wavelength-division multiplexing access (WDMA). Each ONU uses a specific wavelength to carry upstream data. As shown in FIG. 1, n ONUs carry upstream data on optical carriers having wavelengths λ1*, λ2*, . . . , and λn* respectively, and timing and network synchronization are unnecessary. The upstream signals are uploaded through a wavelength grating router (WGR) to a receiving end of the OLT, for example, uploaded to the receiving end of the OLT through an arrayed waveguide grating (AWG) in FIG. 1. But, compared with other broadband access modes, the WDM-PON in the prior art has a very high initial investment and high costs because a lot of light sources need to be used in the system. As shown in FIG. 1, for a system having N ONUs, and each ONU uses upstream and downstream carrier signals having different wavelengths, the WDM-PON system 2N light sources. For some WDM systems, each ONU needs two upstream carriers to obtain high upstream modulation signal power. For a system having N ONUs, each ONU occupies three optical carriers, that is, one downstream carrier signal having a wavelength and two upstream carrier signals having different wavelengths are used, so the system uses 3N light sources. To reduce the number of light sources in the PON system, the following two modes are mainly used in the prior art.
Mode 1: Re-modulation is used at the ONU.
For a system having N ONUs, each ONU uses the same wavelength for transmitting upstream and downstream signals, and N light sources are used in the system, so that the total number of light sources can be reduced to half of the number in the system shown in FIG. 1. Specifically, the ONU divides downstream optical signals carrying downstream data from the OLT into two parts. One part of the downstream optical signals is used for detecting and recovering downstream data. The other part downstream optical signals is used for sending the upstream data, and the upstream data is re-modulated into this part of downstream light and then sent back to the OLT. As shown in FIG. 2, the upstream and downstream data of each ONU are carried at the same wavelength. No light source is disposed at the ONU and the light sources are only disposed at the OLT.
Although the number of light sources can be reduced in mode 1, for the re-modulation mode at the ONU, the downstream data carried at downstream light influences modulation of upstream data, thus influencing reception quality of upstream data at the OLT in the system.
Mode 2: The mode is based on broadband light source (BLS) spectrum splitting.
Two high power BLSs are configured at the OLT. The BLS spectrum splitting method is used to provide downstream optical carriers and upstream optical carriers for the OLT and the ONUs respectively. For the BLSs in FIG. 3, a second BLS is configured to provide wavelength-locked light sources for the OLT, and a first BLS is configured to provide wavelength-locked light sources for the ONUs. After performing spectrum splitting by the demultiplexer (DEMUX) on broad spectrum light beam emitted by the BLSs, a series of narrow spectrum light beams are obtained. The narrow spectrum light beams are injected in a series of receiving/sending portions of the OLTs or ONUs placed inside the CO to generate injection locked light sources, as shown in FIG. 3.
In the spectrum splitting mode, a broad spectrum light beam is split into a plurality of narrow spectrum light beams. Because spectrum gaps exist between the narrow spectrum light beams, power efficiency of the light source is low. In addition, the high power BLS devices are still immature with high costs.
In the prior art, a WDM system needs to use a lot of light sources. Although methods for reducing the number of light sources are provided in the prior art, problems such as low transmission quality of signals, low power efficiency of light source or high costs.